Multi-stage compression system including variable speed motors

ABSTRACT

A multi-stage fluid compression system includes a first centrifugal compressor stage having a first inlet and a first outlet and a second centrifugal compressor stage having a second inlet and a second outlet. The second inlet receives a flow of compressed fluid from the first outlet. A first variable-speed motor is coupled to the first centrifugal compressor stage and is operable to drive the first centrifugal compressor stage at a first speed. A second variable speed motor is coupled to the second centrifugal compressor stage and is operable to drive the second centrifugal compressor stage at a second speed. The first speed and the second speed are each independently variable.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. sec. 119 to provisionalpatent application No. 60/718,389, filed on Sep. 19, 2005, which ishereby fully incorporated by reference.

BACKGROUND

The invention relates to a centrifugal compressor system including twoor more compression stages. More particularly, the invention relates toa centrifugal compressor system that includes multiple compressionstages independently driven by a directly connected high-speed, variablespeed electric motor preferably equipped with active magnetic bearings.

Multiple stage compressor units have been used to provide a higherpressure than would be possible with a single compressor unit. Theseunits are generally driven by a single drive so that all motors operateat a uniform speed or speed ratio.

The use of a single drive motor makes it difficult to vary the operationof one stage with respect to the other stages. For example, a firststage may be operated at an optimal speed under certain conditions.However, this speed may be less than optimal for the other stages. Ifthe stages are driven by a common drive member, the speed of one cannotbe varied without changing the speed of the other.

SUMMARY

In one embodiment, the invention provides a multi-stage fluidcompression system that includes a first centrifugal compressor stagehaving a first inlet and a first outlet and a second centrifugalcompressor stage having a second inlet and a second outlet. The secondinlet receives a flow of compressed fluid from the first outlet. A firstvariable-speed motor is coupled to the first centrifugal compressorstage and is operable to drive the first centrifugal compressor stage ata first speed. A second variable speed motor is coupled to the secondcentrifugal compressor stage and is operable to drive the secondcentrifugal compressor stage at a second speed. The first speed and thesecond speed are each independently variable.

In another embodiment, the invention provides a multi-stage compressionsystem that includes a plurality of centrifugal compressor units. Eachcompressor unit has an inlet and an outlet. A first of the compressorunits draws in a fluid at a first pressure, and a last of the compressorunits discharges the fluid at a second pressure. The compression systemalso includes a plurality of variable-speed motors. Each motor directlydrives one of the plurality of compressor units. Each motor is operableat a speed between a motor minimum and a motor maximum independent ofthe other motors. A control system is operable to vary the speed of eachmotor independently at least partially in response to the secondpressure.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the cross-section of a compression module.

FIG. 2 is a cross-sectional view of a compressor inlet of thecompression module of FIG. 1.

FIG. 3 shows a perspective view of the compression module in ahorizontal configuration.

FIG. 4 is a perspective view of the compression module in a verticalconfiguration.

FIG. 5 is an illustration of the connections and flow from one stage ofcompressor and heat exchanger to another.

FIG. 6 is a schematic of an embodiment of the compression system.

FIG. 7 is a schematic of two motors, one motor driving one compressorand the other motor driving two compressors.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

FIG. 1 illustrates a fluid compression module 10 (sometimes referred toas a compression stage or a compression unit) that includes a primemover, such as a motor 15 coupled to a compressor 20 and operable toproduce a compressed fluid. In the illustrated construction, an electricmotor 15 is employed as the prime mover. However, other constructionsmay employ other prime movers such as but not limited to internalcombustion engines, diesel engines, combustion turbines, etc.

The electric motor 15 includes a rotor 25 and a stator 30 that defines astator bore 35. The rotor 25 is supported for rotation on a shaft 40 andis positioned substantially within the stator bore 35. The illustratedrotor 25 includes permanent magnets 45 that interact with a magneticfield produced by the stator 30 to produce rotation of the rotor 25 andthe shaft 40. In a preferred construction, the rotor is operable at aspeed in excess of 50,000 RPM, with faster and slower speeds also beingpossible. The magnetic field of the stator 30 can be varied to vary thespeed of rotation of the shaft 40. Of course, other constructions mayemploy other types of electric motors (e.g., synchronous, induction,brushed DC motors, etc.) if desired.

The motor 15 is positioned within a housing 50 which provides bothsupport and protection for the motor 15. A bearing 55 is positioned oneither end of the housing 50 and is directly or indirectly supported bythe housing 50. The bearings 55 in turn support the shaft 40 forrotation. In the illustrated construction, magnetic bearings 55 areemployed with other bearings (e.g., roller, ball, needle, etc.) alsosuitable for use. In the construction illustrated in FIG. 1, secondarybearings 60 are employed to provide shaft support in the event one orboth of the magnetic bearings 55 fail.

In some constructions, an outer jacket 65 surrounds a portion of thehousing 50 and defines cooling paths 70 therebetween. A liquid (e.g.,glycol, refrigerant, etc.) or gas (e.g., air, carbon dioxide, etc.)coolant flows through the cooling paths 70 to cool the motor 15 duringoperation.

An electrical cabinet 75 may be positioned at one end of the housing 50to enclose various items such as a motor controller, breakers, switches,and the like. The illustrated embodiment includes a controller 76. Themotor shaft 40 extends beyond the opposite end of the housing 50 toallow the shaft to be coupled to the compressor 20.

The compressor 20 includes an intake housing 80 or intake ring, animpeller 85, a diffuser 90, and a volute 95. The volute 95 includes afirst portion 100 and a second portion 105. The first portion 100attaches to the housing 50 to couple the stationary portion of thecompressor 20 to the stationary portion of the motor 15. The secondportion 105 attaches to the first portion 100 to define an inlet channel110 and a collecting channel 115. The second portion 105 also defines adischarge portion 120 that includes a discharge channel 125 that is influid communication with the collecting channel 115 to discharge thecompressed fluid from the compressor 20.

In the illustrated construction, the first portion 100 of the volute 95includes a leg 130 that provides support for the compressor 20 and themotor 15. In other constructions, other components are used to supportthe compressor 20 and the motor 15 in the horizontal position. In stillother constructions, one or more legs, or other means are employed tosupport the motor 15 and compressor 20 in a vertical orientation or anyother desired orientation.

The diffuser 90 is positioned radially inward of the collecting channel115 such that fluid flowing from the impeller 85 must pass through thediffuser 90 before entering the volute 95. The diffuser 90 includesaerodynamic surfaces 135 (e.g., blades, vanes, fins, etc.), shown inFIG. 2, arranged to reduce the flow velocity and increase the pressureof the fluid as it passes through the diffuser 90.

The impeller 85 is coupled to the rotor shaft 40 such that the impeller85 rotates with the motor rotor 25. In the illustrated construction, arod 140 threadably engages the shaft 40 and a nut 145 threadably engagesthe rod 140 to fixedly attach the impeller 85 to the shaft 40. Theimpeller 85 extends beyond the bearing 55 that supports the motor shaft40 and, as such is supported in a cantilever fashion. Otherconstructions may employ other attachment schemes to attach the impeller85 to the shaft 40 and other support schemes to support the impeller 85.As such, the invention should not be limited to the constructionillustrated in FIG. 1. Furthermore, while the illustrated constructionincludes a motor 15 that is directly coupled to the impeller 85, otherconstructions may employ a speed increaser such as a gear box to allowthe motor 15 to operate at a lower speed than the impeller 85

The impeller 85 includes a plurality of aerodynamic surfaces or blades150 that are arranged to define an inducer portion 155 and an exducerportion 160. The inducer portion 155 is positioned at a first end of theimpeller 85 and is operable to draw fluid into the impeller 85 in asubstantially axial direction. The blades 150 accelerate the fluid anddirect it toward the exducer portion 160 located near the opposite endof the impeller 85. The fluid is discharged from the exducer portion 160in at least partially radial directions that extend 360 degrees aroundthe impeller 85.

The impeller 85 cooperates with a stationary seal ring 162 to define aseal. The seal is positioned to reduce the axial force applied to theback face of the impeller 85, thereby reducing the overall axial thrusttoward the blades 150. The thrust is reduced to a level that allows forthe use of an active magnetic thrust bearing 163 rather than a moreconventional thrust bearing. The magnetic thrust bearing 163 includes athrust disc 164 having a reduced diameter as compared to that whichwould be necessary absent the aforementioned seal system.

The intake housing 80, sometimes referred to as the intake ring, isconnected to the volute 95 and includes a flow passage 165 that leads tothe impeller 85. Fluid to be compressed is drawn by the impeller 85 downthe flow passage 165 and into the inducer portion 155 of the impeller85. The flow passage 165 includes an impeller interface portion 170 thatis positioned near the blades 150 of the impeller 85 to reduce leakageof fluid over the top of the blades 150. Thus, the impeller 85 and theintake housing 80 cooperate to define a plurality of substantiallyclosed flow passages 175.

In the illustrated construction, the intake housing 80 also includes aflange 180 that facilitates the attachment of a pipe or other flowconducting or holding component. For example, a filter assembly could beconnected to the flange 180 and employed to filter the fluid to becompressed before it is directed to the impeller 85. A pipe would leadfrom the filter assembly to the flange 180 to substantially seal thesystem after the filter and inhibit the entry of unwanted fluids orcontaminates.

Turning to FIG. 2, the impeller 85 is illustrated in greater detail. Theinducer portion 155 is substantially annular and draws fluid along anintake path 185 into the impeller 85. The fluid enters in asubstantially axial direction and flows through the passages 175 definedbetween adjacent blades 150 to the exducer portion 160.

FIG. 3 illustrates the compression system or module 10 of FIGS. 1 and 2in perspective. The flange 180 connects to a filter or other source ofclean fluid to receive the fluid to be compressed. In addition, a secondflange 190 is connectable to a pipe, a receiver, or other fluid handlingdevice to receive the compressed fluid from the compression module 10.If the illustrated module 10 were the second stage of a three-stagecompression system, the outlet of the first stage would be connected tothe flange 180 to deliver the partially compressed fluid. After furthercompression, the fluid would be discharged from the second flange 190and would flow to the inlet of the third stage.

FIG. 4 illustrates another compression module 195 in an alternativeorientation, Specifically, the compression module 195 of FIG. 4 issupported in a vertical orientation and is similar to the constructionof FIG. 3 except for the support structure. The construction of FIG. 4includes three legs 200 that support the compression module 195. Ofcourse other constructions may include other support systems and maysupport the compression system 195 in a different orientation ifdesired.

FIG. 5 illustrates a series of compression modules 10 a, 10 b, 10 carranged to define a multi-stage compressor 205. FIG. 5 illustrates eachcompression module 10 a, 10 b, 10 c as being similar to the compressionmodule 10 of FIGS. 1-3. However, other constructions may employ thecompression modules 195 of FIG. 4, may mix the compression modules 10,195 of FIGS. 3 and 4, or use different modules altogether.

For purposes of description, FIG. 5 will be described using air as thefluid being compressed. Of course one of ordinary skill in the art willrealize that many other fluids can be compressed using the presentsystem. The first module 10 a draws in a flow of air 210 in anuncompressed state and discharges a flow of partially-compressed air215. The pressure of the air leaving the first module 10 a is determinedby the inlet pressure and the pressure ratio of the first module 10 a.For example, if air enters the first module 10 a at a pressure of oneatmosphere and the compressor operates at a pressure ratio of 2.5, theair will exit the first module 10 a at a pressure of about 2.5atmospheres.

The partially compressed air 215 flows to an inter-stage heat exchanger220 that cools the partially compressed air 215 to improve the overallcompression system efficiency. In the illustrated, construction, acooling fluid 225 (e.g., cool air, water, glycol, refrigerant, etc.)flows through the heat exchanger 220 to cool the air 215.

A cooled partially compressed air 230 flows into the inlet of the secondstage 10 b of the multi-stage compression system 205. The second stagecompression module 10 b further compresses the air and discharges asecond flow of partially compressed air 235. Again, the dischargepressure is largely a function of the inlet pressure and the pressureratio of the second module 10 b. Continuing the above example, if theair enters the second module 10 b at 2.5 atmospheres and the secondmodule 10 b has a pressure ratio of 2, the discharge pressure will beabout 5 atmospheres.

The second flow of partially compressed air 235 flows through a secondinter-stage heat exchanger 240 where the air is again cooled by acoolant 245 that flows through the heat exchanger 240. After passingthrough the second inter-stage heat exchanger 240, the partiallycompressed air 250 proceeds to the third stage 10 c of compression.

The third stage module 10 c receives the partially compressed air 250 atthe inlet and is operable to further compress the air to the finaldesired output pressure. The air is discharged 255 from the third stagemodule 10 c at the desired output pressure. As with the first two stages10 a, 10 b, the output pressure is a function of the pressure ratio andthe inlet pressure. Thus, finishing the above example, if the air entersthe third module 10 c at a pressure of 5 atmospheres and the pressureratio of the last compressor is 4, the final output pressure will beabout 20 atmospheres.

A final inter-stage cooler 260 may be employed following the final stageof compression to cool the air before the air is directed to additionalsystems (e.g., filters, dryers, etc.) or to a point of use. As with theother heat exchangers 220, 240, a flow of coolant 265 is used to coolthe air before the air is discharged as the final flow of compressed air270. While FIG. 5 illustrates a three-stage system 205 employing asingle compressor at each stage, the present system is well suited insystems that employ more than two or more stages. In addition, somearrangements may include multiple compressors at one or more of thestages to increase the capacity of the system. The multiple compressorsat a given stage may be operated independently or may be operated inunison if desired. As such, the invention should not be limited to athree stage system that employs only a single compressor at each stage.

As one of ordinary skill will realize, the pressure ratio of the threestage system 205 of FIG. 5 is greater than the pressure ratio of any onestage 10 a, 10 b, 10 c. In the example of above, the pressure ratio ofthe three-stage compression system 205 is about twenty to one. Ofcourse, other systems will have different pressure ratios depending onthe desired use or application of the fluid being compressed.

FIG. 6 is a schematic illustration of one possible control arrangement275 suitable for use with the multi-stage system 205 illustrated in FIG.5. Each motor 15 a, 15 b, 15 c includes a motor controller 275 a, 275 b,275 c that directly controls the speed of the attached motor 15 a, 15 b,15 c. A system controller 280 is connected to each motor controller 275a, 275 b, 275 c and provides control signals 285 to each motorcontroller 275 a, 275 b, 275 c to control the speed of the motors 15 a,15 b, 15 c. A first sensor 290 is positioned to measure the outputpressure of the multi-stage system 205 and provides a control signal 295indicative of that pressure to the controller 280. While notillustrated, other sensors could also be employed to send data to thecontroller 280. The data could be used for controlling the motors 15 a,15 b, 15 c or could simply be monitored.

The control scheme 275 illustrated in FIG. 6 allows for the individualcontrol of the speed of each motor 15 a, 15 b, 15 c. Thus, each motor 15a, 15 b, 15 c can be operated at a speed that is suitable for thecompressor 20 a, 20 b, 20 c, while still providing fluid at the desiredpressure and volumetric flow rate. During periods of operation in whichthe operating conditions are not ideal, each motor 15 a, 15 b, 15 c canbe adjusted to run at a speed that produces a suitable flow rate andpressure ratio at the compressor 20 a, 20 b, 20 c, while simultaneouslyproviding the desired conditions for the output fluid.

It can be advantageous to allow motor speeds to differ, to improveefficiency of operation and also to determine when an individualcompressor 20 a, 20 b, 20 c is not operating as it should and possiblyneeds replacement.

FIG. 7 schematically illustrates multiple compression modules 295 a, 295b, including multiple variable speed electric motors 300 a, 300 b. Thefirst compression module 295 a includes a first motor 300 a driving twocompressors 305 a, 305 b and the second compression module 295 bincludes the second motor 300 b and a single compressor 305 c driven bythe second motor 300 b. The two compressors 305 a, 305 b of the firstcompression module 295 a could be positioned in series as illustrated inFIG. 7 or could be arranged in parallel to increase the capacity of thefirst stage of compression.

In operation, each motor 15 is powered by the electrical cabinet 75 andcontroller 76 which rotates the rotor 25 and shaft 40 and ultimatelycauses impeller aerodynamic surfaces 150 to rotate. This draws fluidinto a first compressor 20 via intake path 185 at atmospheric pressureand out through discharge portion 120 at a higher pressure. In multiplestage embodiments, the compressed fluid is sent through a heat exchanger220, 240, 260 which removes some of the heat that has been produced bycompressing the fluid. This cooled fluid is then drawn into a secondcompressor 20 and to any number of desired additional stages to allowfor different pressures of fluid to be available for use, or to attain agreater pressure than would normally be possible by a single compressor.

The controller 280 includes information about compressed gastemperatures and pressures, positions of valves, stability margin of thecompressor 20 a, 20 b, 20 c, requirements of the system upstream thecompressor 20 a, 20 b, 20 c, and performance parameters of the auxiliarysystems. The speed of each motor 15 a, 15 b, 15 c is controlled andvaried by the controller 280 in response to the desired output pressure,ambient temperature and pressure, fluid temperature and other relevantvariables as may be required. For example, one construction includes apressure sensor and a velocity sensor at the outlet of each compressionstage. The pressure and velocity are used to determine the volumetricflow rate of the stage and the pressure is used to determine thepressure ratio. These values are then applied to a compressor map, alongwith the speed of the compressor, to determine if the compressor hassufficient surge margin and choke margin. Each stage of compression isoptimized to run efficiently, with sufficient margins, and at a speedthat allows for the output of compressed fluid having the desiredcharacteristics (e.g., pressure, flow rate, etc.).

The use of a high-speed motor 15 a, 15 b, 15 c directly coupled to thecentrifugal compressor 20 a, 20 b, 20 c eliminates the required gearingand related oil lubrication requirements of a non-directly connectedsystem. In recent years, high-speed motor technology, as applied tooil-free air centrifugal compressors, has evolved considerably. Activemagnetic bearings which levitate the shaft in air are the bearing systemoften utilized in high-speed electric motors, since they introduce asignificant power loss advantage with respect to the application ofconventional fluid-film hydrodynamic bearings.

The development of an industrial multistage centrifugal compressorsystem 205 with independently directly driven compressor modules 10 a,10 b, 10 c is advantageous because of the de-coupling between the speed,the location, and the operating mode of the stages of compression 10 a,10 b, 10 c. Furthermore, high-speed synchronous electric motors 15 a, 15b, 15 c are operable to vary the rotational speed of the compressorstages 10 a, 10 b, 10 c and can satisfy, in terms of overall compressorstability and overall power consumption, the demand of the downstreamprocess.

The de-coupling of the drive member, and therefore the speed, location,and operation of the various stages also eliminates the need for aconventional inlet valve and can eliminate the need for dump valves andother flow control systems that are typically used to avoid operatingone or more stages near the surge limit. The elimination of thesefeatures reduces the cost and complexity of the system and improves theoverall efficiency.

It should also be noted that the design of the impeller 85 as well asthe shaft 40 and the active magnetic bearings 55 is such that therotating assembly operates below its first critical speed under allnormal operating conditions. The sub-critical operation is achieved byproviding a light yet stiff rotating assembly. To achieve this, theimpeller 85 is compact for it's size and is formed using a light yetstrong material (e.g., titanium alloys, aluminum, etc.).

Various features and advantages of the invention are set forth in thefollowing claims.

1. A multi-stage fluid compression system comprising: a firstcentrifugal compressor stage having a first inlet and a first outlet; asecond centrifugal compressor stage having a second inlet and a secondoutlet, the second inlet receiving a flow of compressed fluid from thefirst outlet; a first variable-speed motor coupled to the firstcentrifugal compressor stage and operable to drive the first centrifugalcompressor stage at a first speed; and a second variable speed motorcoupled to the second centrifugal compressor stage and operable to drivethe second centrifugal compressor stage at a second speed, the firstspeed and the second speed each being independently variable.
 2. Themulti-stage fluid compression system of claim 1, wherein one of thefirst centrifugal compressor stage and the second centrifugal compressorstage includes a first shaft that supports a first impeller and a secondimpeller, and the other of the first centrifugal compressor stage andthe second centrifugal compressor stage includes a second shaft thatsupports a single third impeller.
 3. The multi-stage fluid compressionsystem of claim 1, wherein the first centrifugal compressor stageincludes a first shaft that supports a first impeller and a secondimpeller, and the second centrifugal compressor stage includes a secondshaft that supports a third impeller and a fourth impeller.
 4. Themulti-stage fluid compression system of claim 1, further comprising aheat exchanger configured to receive the flow of compressed fluid fromthe first outlet and deliver the flow to the second inlet.
 5. Themulti-stage fluid compression system of claim 1, further comprising acontrol system operable to control the speed of each of the motorsindependently.
 6. The multi-stage fluid compression system of claim 5,further comprising at least one sensor associated with the firstcentrifugal compressor stage and at least one sensor associated with thesecond centrifugal compressor stage, the control system operable todetermine an output flow rate and an output pressure of the firstcentrifugal compressor stage and the second centrifugal compressor stagebased at least partially on the data sensed by the sensors.
 7. Themulti-stage fluid compression system of claim 6, wherein at least one ofthe sensors measures a pressure and at least one of the sensors measuresa velocity.
 8. The multi-stage fluid compression system of claim 1,further comprising a first motor controller operable to control thespeed of the first motor and a second motor controller operable tocontrol the speed of the second motor.
 9. The multi-stage fluidcompression system of claim 1, further comprising an active magneticthrust bearing coupled to the first centrifugal compressor and operableto support the thrust load of the first centrifugal compressor.
 10. Themulti-stage compression system of claim 1, wherein each motor includes ashaft that rotates at a speed that is greater than or equal to about50,000 RPM.
 11. A multi-stage compression system comprising: a pluralityof centrifugal compressor units, each compressor unit having an inletand an outlet, a first of the compressor units drawing in a fluid at afirst pressure and a last of the compressor units discharging the fluidat a second pressure; a plurality of variable-speed motors, each of themotors directly driving one of the plurality of compressor units, eachmotor operable at a speed between a motor minimum and a motor maximumindependent of the other motors; and a control system operable to varythe speed of each motor independently at least partially in response tothe second pressure.
 12. The multi-stage compression system of claim 11,wherein each of the plurality of centrifugal compressor units includes ashaft that supports a first impeller for rotation, and wherein at leastone of the shafts supports a second impeller in addition to the firstimpeller.
 13. The multi-stage fluid compression system of claim 11,further comprising a plurality of heat exchangers, each heat exchangerdisposed between the outlet and the inlet of adjacent centrifugalcompressor units.
 14. The multi-stage fluid compression system of claim11, further comprising a control system operable to control the speed ofeach of the motors independently.
 15. The multi-stage fluid compressionsystem of claim 14, further comprising a plurality of sensors, eachsensor associated with at least one of the centrifugal compressor units,the control system operable to determine an output flow rate and anoutput pressure of each of the centrifugal compressor units based atleast partially on the data sensed by the sensors.
 16. The multi-stagefluid compression system of claim 15, wherein at least one of thesensors associated with each compressor unit measures a pressure and atleast one of the sensors associated with each compressor unit measures avelocity.
 17. The multi-stage fluid compression system of claim 11,further comprising a plurality of motor controllers, each operable tocontrol the speed of one of the plurality of motors.
 18. The multi-stagecompression system of claim 11, further comprising a plurality of activemagnetic thrust bearings, each magnetic thrust bearing coupled to one ofthe plurality of centrifugal compressor units and operable to supportthe thrust load of the associated centrifugal compressor unit.
 19. Themulti-stage compression system of claim 11, wherein each motor includesa shaft that rotates at a speed that is greater than or equal to about50,000 RPM.
 20. The multi-stage compression system of claim 19, furthercomprising a plurality of impellers, each impeller coupled to one of theshafts, each shaft supported for rotation by at least two activemagnetic bearings.